Thermoplastic polymers such as poly(ethylene terephthalate) and poly(butylene terephthalate) are produced on a large scale in industry. To prepare these useful polymers, generally either the appropriate dicarboxylic acids are esterified directly with diols, or dicarboxylic esters, such as dimethyl terephthalate, are transesterified with diols. Such polymerization methods are well known. See, for example U.S. Pat. Nos. 4,339,570 (Muschelknautz et al.), 4,154,920 (Jabarin), 3,644,294 (Siclari et al.), and Swiss Patent Application 237,400.
Esterification of carboxylic acids and the transesterification of carboxylic acid esters are common and useful chemical reactions. Such reactions can be useful in the preparation of useful thermoplastic polyesters such as poly(ethylene terephthalate) and poly(butylene terephthalate). See, for example U.S. Pat. Nos. 5,064,935 (Jackson et al.), 4,973,655 (Pipper et al.), 3,509,203 (Mechaelis et al.), and 3,167,531 (Parker et al.).
Certain byproducts of esterification or transesterification are generally undesirable because of their effects on resulting polyesters. See, for example, Kumar, et al., "Optimization of the Transesterification Stage of Polyethylene Terephthalate Reactors," Polymer Engineering and Science, Vol. 24 (No. 3), pp. 185-193, (Feb. 1984), and Patkar and Jabarin, "Effect of Diethylene Glycol (DEG) on the Crystallization Behavior of Poly(ethylene terephthalate) (PET)," Journal of Applied Polymer Science, Vol. 47, pp. 1749-1763 (1993).
The Mechaelis et al. patent discloses a transesterification process comprising passing the mixture of reactants through narrow annular channels, the width of the channels with respect to their depth being smaller than 1.5:1 (see col. 3, lines 41-44). The working examples had a mean dwell time of from 200 to 240 minutes.